Effects on soil and alternatives for biological control of the invasive plant carpobrotus edulis

Resumo

Carpobrotus edulis is a South African plant, highly invasive in coastal temperate areas. It establishes in dunes, rocky areas or cliffs, where it can alter the biotic and abiotic factors of the ecosystems. The effects of C. edulis on soils are mediated by its necromass, which differs in chemical composition from that of native vegetation. Depending on the amount of necromass accumulated, C. edulis can have contrasting effects on soil pH, which lead to variations in the effects on macro- and micro- nutrient availability. The differences in necromass composition between native plants and C. edulis may reflect competitive advantages of the exotic through lower accumulation of toxic elements (Al), higher content of some macronutrients (Mg, Ca), lower requirement (or higher resorption from senescence tissues) of some micronutrients (Co, Cu, Fe, Ni, Zn) and Na accumulation (as a strategy against salinity). Carpobrotus edulis can also alter the soil N cycle through changes in N fluxes (higher ammonium immobilization and lower autotrophic nitrification and dissimilatory nitrate reduction to ammonium) that can limit N availability. Currently, this plant is controlled mostly with mechanical or chemical methods, which cannot be implemented in areas with erosion risk, with endemic or endangered species or with difficult access (cliffs). However, potential agents for biocontrol had not been studied thus far. The fungus Sclerotinia sclerotiorum can infect C. edulis, but with no long-term consequences in greenhouse conditions. The insect Pulvinariella mesembryanthemi can reduce survival and growth of the plant. The combined use of both agents has no long-term synergic effects and is detrimental for the insect establishment. Pulvinariella mesembryanthemi was introduced accidentally in many C. edulis invaded areas around the world including NW Spain, where its modelled potential distribution roughly overlaps that of C. edulis. However, the abundance of P. mesembryanthemi in this area may be restricted by low winter temperatures, parasitism and predation. These abiotic and biotic constraints, jointly with a limited dispersal, may explain the more restricted distribution of P. mesembryanthemi than C. edulis. In NW Spain, other enemies of C. edulis are naturally found (predators, parasites, diseases), but with no apparent effect on the plant populations. Pulvinariella mesembryanthemi introductions in Western Europe and Oceania (genetically very homogeneous) seem to originate from the same native population, and most of the genetic variability of the insect resides in its native area. The insect is associated with an alpha-Proteobacteria endosymbiont (both in the native and exotic populations), with very low genetic variation.